deathpectation
My wrong before I phrased the title bad, haven't tested with simcards. But it turns out it works fine, just tested with two different sim card, so I don't think its a Verizon thing.
Sorry, maybe I didn't phrase it very clearly. If both SIM cards work, it just means that your phone is not carrier locked.
Nevertheless, it may still be the case that the bootloader cannot be unlocked (OEM-unlock). It sounds like Verizon is your carrier, and if I interpret the forum posts correctly, it seems to be the rule rather than the exception that the bootloader from phones purchased from Verizon can't be unlocked (I don't use Verizon, so I can't confirm this myself).
but it's seems this security feature is more common among google pixel phones...
Actually most Android devices ship with a locked bootloader. A locked bootloader is an important security feature (therefore it should be locked again directly after the installation of GrapheneOS. Full Verified Boot will only work when the bootloader is locked (either with stock OS (green Boot state) or an alternate OS with a custom root of trust (yellow Boot state)).
This steps are not described in details on GOS site, where can I find instruction about this
If I understand the GOS instructions mentioned above correctly, the system should be able to automatically check whether it allows OEM unlock or not when accessing the Internet. Unfortunately, I can't answer this more precisely as I haven't had this problem.